1. Field of the Invention
The present invention relates generally to an organic electroluminescent device having an organic layer including luminescent materials provided between a hole injection electrode and an electron injection electrode, and an organic electroluminescent apparatus using such an organic electroluminescent device. More particularly, it relates to an organic electroluminescent device capable of emitting light in various colors and an organic electroluminescent apparatus capable of simply changing the color of light emitted in the organic electroluminescent device and simply adjusting the luminance thereof.
2. Description of the Related Art
In recent years, the needs of flat panel display devices the consumed power and the size of which are smaller than those of a CRT (cathode-ray Tube) which has been conventionally generally employed have been increased as information equipments are diversified, for example. Therefore, an electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided into an inorganic electroluminescent device and an organic electroluminescent device depending on a used material.
The inorganic electroluminescent device is so adapted that a high electric field is generally exerted on a luminescent portion, and electrons are accelerated within the high electric field to collide with a luminescence center, so that the luminescence center is excited to emit light. On the other hand, the organic electroluminescent device is so adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other to bring an organic molecule into its excited state, and the organic molecule emits fluorescence when it is returned from the excited state to its ground state.
In the case of the inorganic electroluminescent device, a high voltage of 100 to 200 volts is required as its driving voltage because a high electric field is exerted as described above. On the other hand, the organic electroluminescent device can be driven at a low voltage of approximately 5 to 20 volts.
Therefore, in recent years, various studies have been conducted on the organic electroluminescent device. A three-layer structure referred to as a DH structure obtained by laminating a hole transporting layer, a luminescent layer and an electron transporting layer between a hole injection electrode and an electron injection electrode, a two-layer structure referred to as an SH-A structure obtained by laminating a hole transporting layer and a luminescent layer abundant in electron transporting properties between a hole injection electrode and an electron injection electrode, and a two-layer structure referred to as an SH-B structure obtained by laminating a luminescent layer abundant in hole transporting properties and an electron transporting layer between a hole injection electrode and an electron injection electrode have been developed as the organic electroluminescent device.
The organic electroluminescent device can be caused to emit light in a suitable color by suitably selecting its luminescent material. Therefore, it is expected that the organic electroluminescent device can be also utilized as a multi-color display device, a full-color display device, and the like.
Conventionally when the organic electroluminescent device is caused to emit light, one organic electroluminescent device emits light in one type of color.
When multi-color or full-color display is performed using a conventional organic electroluminescent device, therefore, it is necessary to use a plurality of organic electroluminescent devices which differ in luminescent color in combination. When full-color display is performed, three types of organic electroluminescent devices emitting light in the three primary colors, that is, blue, green and red are used in combination.
In order to combine three types of organic electroluminescent devices to perform full-color display, however, three types of wiring for supplying a current to the organic electroluminescent devices must be provided, resulting in a complicated wiring method.
Since the three types of organic electroluminescent devices emitting light in blue, green and red are used as one unit to perform full-color display, the size of a pixel for performing full-color display is approximately three times a pixel in a single color. Therefore, it is difficult to display a highly precise full-color image particularly on a small-sized screen.
In recent years, in causing an organic electroluminescent device to emit light, a method of changing a voltage to be exerted on a portion between an electron injection electrode and a hole injection electrode in the organic electroluminescent device and a method of changing the duty ratio of a voltage to be applied have been proposed in order to adjust the luminance thereof.
In these methods, the luminance of light emitted from the organic electroluminescent device is merely adjusted, as described above. In one organic electroluminescent device, light in one type of color is only emitted. When multi-color or full-color display is performed, a plurality of organic electroluminescent devices which differ in luminescent color, as described above, must be used in combination, so that the above-mentioned problems still exist.